The tin alloy plating bath (solution) used to form a tin alloy plating film on electroconductive objects, for example, a tin-silver alloy plating film, readily forms salts of more noble metal ions than tin that are insoluble in plating bath and so readily deposit when the oxidation/reduction potential of metal ions other than tin ions in the bath (for example, silver ions) differs greatly. Thus, maintenance of a stable bath is known to be difficult. Consequently, plating solutions that contain cyanide have been used in the past as tin-silver alloy plating solutions. However, this bath is extremely toxic because it contains toxic cyanide, and various problems are associated with its handling.
Tin-silver alloy plating baths that contain thiourea or thiourea derivatives Japanese Kokai publication Hei-9-302498, tin-silver alloy plating baths that contain thiol compounds such as mercaptosuccinic acid Japanese Kokai publication Hei-9-170094, or tin-silver alloy plating baths that contain aliphatic sulfides or aliphatic mercaptans Japanese Kokai publication 2006-265572 have been disclosed as tin alloy plating baths that do not contain cyanide.
However, experiments by the inventors have revealed that silver in these solutions is not capable of stable, long-term dissolution. The silver precipitates immediately after preparation of the plating bath or within 24 hours following preparation of the plating bath. This so-called bath decomposition precludes the long-term, stable use of a plating bath. In addition, the ratio of tin and other metals in a tin alloy precipitate varies greatly with change in the current density during electroplating, and a stable precipitation rate has been impossible to maintain.
Consequently, the development of a tin alloy plating bath with high serial stability that does not contain cyanide has long been desired.